Revert "gpio: include <linux/io-mapping.h> in gpiolib-of"

This reverts commit 7d4defe2.

The commit was pointless, manically trembling in the dark for
a solution. The real fixes are:

commit 048c28c9
("gpio: make any OF dependent driver depend on OF_GPIO")
commit 2527ecc9
("gpio: Fix OF build problem on UM")

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>